This invention relates to an amplifying unit capable of amplifying a broadband input signal and, in particular, to an amplifying unit comprising an input transformer and a broadband amplifier.
In recent years, in the cable television (CATV) industry or the like, there is a demand to receive a lot of channels, for example, up to one hundred image channels. In compliance with the demands for a lot of channels, it is necessary to provide a broadband amplifier which is capable of amplifying an input signal uniformly and with low distortion that ranges from a low frequency of about 50 MHz to a very high frequency of about 1 GHz. In other words, to extend over a wide frequency band of about 700 MHz to 1 GHz.
Here, the amplifying unit for the CATV of the type described normally comprises not only the above-mentioned broadband amplifier but also an input transformer and an output transformer at input and output sides of the broadband amplifier in connection with impedance matching for a cable or other equipment.
In the prior art, for the amplifying unit having such a structure, miniaturization is strongly needed in a manner similar to other circuits. However, it is difficult to sufficiently miniaturize the amplifying unit. This is because the input transformer and the output transformer cannot be implemented by an integrated circuit (IC) chip. The broadband amplifier can be implemented by an IC chip and results in miniaturizing. On the other hand, inasmuch as the input transformer and the output transformer for use in the amplifying unit of the type described must have the impedance matching capability over the wider frequency band, various devices have been made therefor. However, no proposal has yet been made for a transformer which is inexpensive and which maintains an impedance match over the wide frequency band required for CATV. Accordingly, so long as a conventional proposed input transformer and output transformer are used, it is impossible to avoid impedance mismatching and it is impossible to prevent degradation of image quality due to generation of a ghost signal accompanied with generation of a reflected wave and the degradation of efficiency in transmission. In particular, the adverse affects increase even with a little impedance mismatch in the CATV supply signals to a broad area, inasmuch as the CATV comprises a lot of broadband amplifying units. As described above, the impedance matching in the input and the output transformers is a very significant problem in the amplifying unit used in CATV.
Various amplifying units of the described type are already known. By way of example, a hybrid amplifier is disclosed in U.S. Pat. No. 4,965,526 (hereinafter referred to as reference 1) issued to Scott Craft et al. on Oct. 23, 1990, which is hereby incorporated herein by reference. The hybrid amplifier of reference 1 operates in the radio frequency (RF) range of 100-1000 MHz or higher. In reference 1, active elements composing the amplifier are implemented by chips, and the chips and input and output transformers are mounted on a circuit board. In addition, symmetrical parts of the electrical circuit are arranged in a mirror symmetrical fashion. With this structure, it is possible to reduce the adverse affects of parasitics related to the electrical circuit.
Furthermore, reference 1 uses, as the input and the output transformers, magnetic coupled transformers through which connection between a transmission line and the amplifier and connection between an external circuit and the amplifier are carried out. Each magnetic coupled transformer comprises a toroidal core and a winding wound around the toroidal core. In addition, the magnetic coupled transformers are arranged on the substrate in a planar fashion and each winding of the magnetic coupled transformer has terminals which are individually connected to a metal pattern or a conductor pattern arranged on the substrate.
A high frequency linear amplifier is disclosed in U.S. Pat. No. 5,142,239 (hereinafter referred to as reference 2) issued to Daniel C. Brayton et al. on Aug. 25, 1992, which is hereby incorporated herein by reference. The high frequency linear amplifier comprises a broadband amplifier and a transmission-line transformer connecting the broadband amplifier and a transmission line. In addition, reference 2 uses a set of two-wired parallel lines having a predetermined characteristic impedance and teaches use of the transmission-line transformer. With this structure, it is possible to improve frequency characteristics compared to reference 1, and to achieve miniaturization and light weight.
A balance/unbalance converter is published in Japanese Unexamined Patent Publication of Tokkai No. Hei 7-240,652 or JP-A 7-240,652 (hereinafter referred to as reference 3) on Sep. 12, 1995, which is hereby incorporated herein by reference. The balance/unbalance converter comprises a transmission-line transformer between a balanced cable and an unbalanced cable and two resistor elements which are connected between the balanced cable in series. A middle point of the two resistor elements is grounded. With this structure, a middle potential is fixed by grounding the middle point of the two resistor elements and results in reducing the imdemence in a common mode.
However, the above-mentioned references 1 through 3 have problems described as follows. In the reference 1, the magnetic coupled transformers occupy a large area in the substrate because the magnetic coupled transformers are arranged in a planar fashion. In addition, inasmuch as each winding of the magnetic coupled transformer is wired to the conductor pattern or the metal pattern and has a long wire distance between the toroidal core and the conductor pattern, it is disadvantageous in that because a parasitic inductance changes by dispersion due to wiring, it is necessary to finely adjust after wiring, thereby degrading the frequency characteristics. After adjustment, it is necessary to fix the toroidal core and the wiring to the substrate by an adhesive such as varnish. On fixing, the float capacitance changes due to dielectric constant of the adhesive and results in the problematic shifting from an adjusted characteristic. As described above, much time is consumed in adjusting. In addition, when magnetic coupled transformers are used, a higher frequency band is restricted by a coupling coefficient between lines and a lower frequency band is restricted by the material of the core. As a result, the magnetic coupled transformer has a bad frequency characteristic and is unsuitable for the wider frequency band.
In addition, reference 2 may teach utilization of a transmission-line transformer and a two-wire parallel line, but it does not take into account the connection relationship at the input and output sides of the transmission-line transformer suitable for the broadband amplifier. Therefore, reference 2 only takes account of selection so that a ratio of input and output impedances is equal to 1:n.sup.2 (concretely 1:1) in a manner similar to a normal transmission line transformer. Accordingly, it is difficult for the reference 2 to realize other impedance ratios.
At any rate, when the transmission-line transformer having an impedance ratio of 1:1 is connected to the balance input of the amplifier, it is disadvantageous in that a middle point is unsettled, characteristics such as a gain characteristic of the high frequency amplifier and a characteristic of a secondary distortion are unstable. In addition, when a small-sized core is used to meet a of miniaturization requirement, a ground impedance becomes small and the adverse affects of signals of even mode (which is called common mode) are increased.
In addition, inasmuch as reference 3 also does not teach a connection relationship between the transmission-line transformer and the broadband amplifier, it is difficult for the reference 3 to determine the connection relationship between the transmission-line transformer and the broadband amplifier suitable therefor and a structure therefor. Furthermore, inasmuch as high frequency power reversed in phase 180 degrees is terminated via a terminal resistor at the output port of the transformer in the structure of reference 3, termination is made at a constant impedance under any frequency. However, it is difficult for reference 3 to fix the middle potential without changing the impedance ratio between the input side and the output side. This is because of the following reasons. When the terminal resistor has a large value, a little high frequency current flows in the terminal resistor, and it is therefore impossible to stably maintain the middle potential. When the terminal resistor has a small value, the conversion ratio varies.